Osteoporosis and vascular disease affect large numbers of aging individuals, and there is increasing evidence that the two disorders are closely related. Both animal and human data indicate that osteoporosis is associated with vascular disease and calcification. Moreover, the process of vascular calcification appears to involve pathways very similar to those utilized during normal bone formation. Circulating endothelial progenitor cells (EPCs) are now increasingly recognized as playing an important role in initiating vascular repair in response to endothelial injury. Our recent demonstration that patients with vascular disease have a 5- to 8-fold greater percentage of EPCs expressing an osteogenic phenotype, as assessed by staining for the bone- related protein, osteocalcin (OCN), has led us to propose the overall novel concept that expression of an osteogenic phenotype by circulating EPCs may explain, at least in part, the increase in vascular calcification that occurs with aging and in the setting of atherosclerosis. In the present, multi-disciplinary proposal utilizing the multiple PI mechanism (Dr. Lerman [Vascular Disease, Cardiology] and Dr. Khosla [Bone Metabolism, Endocrinology]), along with the help of an expert in Vascular Biology (Dr. Simari) as Co-Investigator, we will test this hypothesis within the context of our three Specific Aims: Aim 1: Using purified EPCs from patients undergoing cardiac catheterization, test the hypotheses that, as compared to normal control subjects, circulating EPCs from patients with established CAD express an osteogenic transcriptosome (i.e., multiple bone-related genes/pathways) and have an increased ability to form mineralized nodules and/or enhance mineralization of vascular smooth muscle cells in vitro. Aim 2: In in vivo studies using murine models, test the hypotheses that that local delivery of EPCs expressing an osteogenic phenotype (defined by staining for the osteoblastic marker, AP), as compared to EPCs negative for AP staining, leads to increases in both subcutaneous bone formation (Aim 2a) and vascular calcification (Aim 2b).and Aim 3:In clinical studies in patients with non-obstructive CAD undergoing coronary cardiac catheterization, test the hypotheses that the expression of osteogenic/mesenchymal markers (OCN, AP, Stro-1) by systemically circulating EPCs correlates with early atherosclerosis (as defined by endothelial dysfunction) and with vascular calcification (as assessed by the sensitive method of intravascular ultrasound [IVUS]) (Aim 3a), and that these osteogenic EPCs are being consumed by the coronary circulation (Aim 3b). The proposed studies thus test novel hypotheses using a multidisciplinary approach. Moreover, the results of these studies are likely to have significant implications for our understanding of the mechanisms initiating vascular calcification as well as the increasing pathophysiological link between the vasculature and bone metabolism.